Electron multiplier



Feb. 22, 1955 G. HERZOG ELECTRON MULTIPLIER Filed April 2, 1949 15 v MourPur G/PHA/P Ll By M IN V EN TOR. 2%200 ATTORNEY? #250 VOL rs "/500VOL 75 United States Patent I O 2,702,865 ELECTRON MULTIPLinii GerhardHerzog, Houston, Tex., ass'i'giior toTlle Texas Company, New York, N.Y., a corporation of Delaware Application April 2, 1949, Serial No.85,174

6 Claims. (Cl. 250 207 This invention relates to' electron multipliersof the photocell type, usually termed multiplier tubes.

Such multiplier tubes I envelope containing a photocathode, an anode,and one or more so-called target electrodes or dynodes between thecathode and the anode. As distinguished from the simple photocellcontaining only a cathode and an anode, the photo electrons emitted bythe cathode by reason of the radiation directed the'reagainst, insteadof passing to the anode, pass to a target electrode or dynode which ismaintained at a positive potential with respect to the cathode. Theseprimary electrons from thecathode cause the emission of secondaryelectrons in greater number from the target electrode or dynode, the.total'nurnber of electrons discharged from the target electrode. beinggreatly in excess of the primary electrons striking the electrode,thereby effecting a multiplication of theorigie nal electronic impulses.By setting up a series of such target electrodes or dynodes atsuccessively higher positive potentials, the electron stream can becaused to impinge successively on the electrodes, the electron stream.being greatly increased by its contact with each target electrode,thereby obtaining a final multiplication factor which is a function ofthe factor S of eachtarget electrode and of the number n of targetelectrodes, i. e.,

Such multiplier photocells have found a number of uses, a typical usebeing the measurement of radiation emitted from a phosphor such asnaphthalene or scheelit'e when subjected to penetrative'radiation suchas gamma rays. In such a situation, the gamma rays striking: thephosphor cause scintillations in the ultraviolet and near visible rangeof the'spectrum' which are observed by the photocathode. While theobserved scintillation's may be of small magnitude, the photocellmultiplies the" sciutula: tions or the electrons produced thereby to anextent as to be amplified and measured by more conventional andrelatively simple apparatus'such as an amplifier in combination with apulse height discriminator and a rate meter. A typical apparatus isdescribed in Nu'cle'otiics, January 1949, pp. 16 et seq.

Inthis application and in other applications," such multiplierphotocells are characterized by a so-called dark current which causesthe emission of electrons internally within the cell, this emissioninterfering with the use of the cell, and contributing to undesirablenoise therein. At timesjthis noise exceeds the magnitude due to thephotoelectrons and prevents measure ments of any type. v

There are two causes of the dark current. One. cause is internal orexternal leakage between the cathode and the anode. This leakage of thecurrent is not considered to present a difi'icult problem and canusually be corrected by mounting the cathode terminal as far as possiblefrom the anode terminal; A second and perhaps the principal cause is thethermionic emission inherent in the photocell. At room temperature, thisemission by itself may be of a magnitude in excess of the radiation oremission being measured. The thermionic emission can be reduced bycooling the cell to a temperature of about -40 C. Obviously this is nota practical solution. Coincidence circuits have been proposed as asolution but they require the use of more than one photocell and addedcircuits. proportional to the photocathode area, it can be reducedordinarily comprise a vacuum Since the thermionic e'miss'i'onis roughlyat page 79 of Photoelectric 2 byreducing the cathode area. desirable. vv p The present invention has as itsmajor object the .provision of anovelphotocell wherein the thermionic emissionis substantiallyeliminated in a practical manner without resorting to any of the abovedevices. v v

A further object of the invention is the provision ,ofa photocellwherein the photocathode can be made. ,as large as desired without aprohibitive increase in therinionic emission anda resultant noise. v

Still a further object of the invention is the provision of a novelmethod of operating a photocell wherein the noisei due to thermionicemission is substantially eliminate I Other objects of the inventionwill appear fromjhe following description and claims taken in connectionwith the attached. drawing which illustrates an embodiment of theinvention showing a photocell in combination with a phosphor and asource of penetrative radiation.

In brief, the present invention can be described as involving a methodand means whereby thermionic electrons generated within a multiplierphotocell vare suppressed therein and their flow to the respectivetarget electrodes is halted. Preferably this is accomplished by the useof one or more grid suppressors in front of one or more targetelectrodes, the grid or grids being impressed witha voltage whichsuppresses the thermionic electrons and passes the photo electrons whichit is desired to measure. v p h The invention can be ilustrated byreference to the drawings wherein a practical application of amultiplier photocell is illustrated diagrammatically. In the drawing,Fig. 1 is a representation of a scintillometer usinga photofmultip'liertube which ismodified accordingto the presentinvention, and is shown intransverse cross se ction, and Fig. 2 is a schematic diagram of asuitable cir; cuit for the photomultiplier tube. In Fig. 1 S is a sourceof penetrative radiation such as gamma rays, P, is a so; called phosphorsuch as naphthalene or synthetic or But, this is not always .naturalcalcium tung'state which under the bombardment of .gammarays, eectrons,and alpha particles emitsradiation in. the near visible and ultravioletranges of the spectrurri and T is a diagrammatic representation of amultiplier photocell.

Photocell T, of generally conventional type as shown Cells by Sommer(Chemical Publishing Company, Inc., 1 947), includes a- PhOtQe cathode11' exposed to radiation from phosphor P,,,a1i anode 12, arranged to beconnected to a suitable amplifyingmeans and measuring means, and seriesoftargetelec- I trodes or dynodes 13 between the cathode and theanode.-.

Eachof said target electrodes has a secondary emitter surfaceorsensitized side as is customary in conventional types of photomultipliertubes.

In operation, phosphor or luminophor P is exposed to bombardment ofradiationfrom source S, the phosphor then emitting radiation which isdirected onto photocathode 11, causing the emission of electrons whichpass from target electrode to target electrode in the cell by reason ofthe successively higher potentials, impressed on the respectiveelectrodes, pulses being finally obtained at anode 12'. The numbermeasure of the intensity of the original penetrative radiation. Thepresent invention contemplates the use of a ,veloc; ity filter grid14which is supported on a pair of support ro'ds 14a between photocathode11 and targetelectrode 13a, the grid being maintained at a negativevoltage with respect to' the photosensitive or photo-emissiye cathode 11(see Fig. 2), that voltage being substantially the emitted thermionicelectrons. As in conventional practice, aresistor or a series ofindividual resistors, are connected in shunt across the direct currentsupply source as shown in Fig.

2. A special connection is, however shown in this figure connecting thefilter grid 1'4 to the most negative ter';

minal' of the'resistor system; thus to provide a source filter grid ofdirect voltage between cathode 11 and the v 14, with the latterconnected to the negative pole of the" main supply source to polarize itnegatively with.

of the pulses constitutes a.

respect to the cathode. Thus the thermionic electrons will be hinderedfrom passing the grid while the photoelectrons with their higher energywill pass through the grid and through the remainder of the multiplier.Stated otherwise, the grid affords electrical means whereby adiscrimination can be effected between the photoelectrons and thethermionic electrons. One of the support rods 14a may be conductivelyextended, in a direction corresponding to downward in the drawing,through the press and base of the tube to an appropriate one of itsterminal pins, whereby this support rod may serve to carry an externallyapplied polarizing potential to the filtering grid 14. Since the base,the press and the terminals are included among conventional parts of thetube which may be made in accordance with the prior art and do not ofthemselves constitute features of the present invention, they are notshown or described in detail herein to simplify the drawing and thedisclosure.

If desired, this principal of such a grid in front of a dynode may beextended to other than the first dynode 13a, the voltage on each suchgrid being necessarily different. Thus a velocity filter grid, like thegrid 15 shown in Fig. 1, may be mounted on a pair of support rods 15aclosely adjacent and parallel to the secondary emitter surface of thedynode 13a athwart the path between it and the second dynode, i. e., infront of the second dynode. However, such use of a grid in front ofother than the first dynode is not necessary to a material reduction ofnoise since the electrons from the other dynodes are subject to lessoverall multiplication and, therefore, are of less effect.

The action of the grid or grids may be explained as follows: Theelectrons which leave a metal surface by thermionic emission have anaverage kinetic energy of ZkT where k is Boltzmanns constant or 1381x10-ergs per degree centigrade which is the same as 7/ 80,000 electron voltsper degree centigrade, and T is the temperature in degrees Kelvin (seepages 112-113 and 708 of Introduction to Modern Physics, 3rd Edition, byRichtmyer and Kennard, published by McGraw-Hill Book Co., 1942). Thus,for a surface at room temperature, the average kinetic energy of thethermionic electrons is 0.05 electron volt.

On the other hand, electrons are emitted due to the action of thephotons which are released from the phosphor under the bombardment ofthe penetrative radiation. The energy of these photons can be determinedfrom the wave length according to the formula where A is the wave lengthof the photons in Angstrom units. The photoelectric electrons will losean energy equal to the work function of the metal and will loseadditional energy in collisions before they manage to leave the cathode.They will have a maximum possible kinetic energy outside the cathodewhich is given by Energy V ID8X A where W is the work function of thecathode material.

According to available authorities, the wave length of the fluorescentlight of naphthalene is between 3200 and 4200 A. Assuming an averagevalue of 3700 A, these quanta will produce electrons of an energy of3.3-W electron volts. The work function of cesium which is widely usedas one of the components of a photo cathode surface is 1.9 volts.Therefore, the maximum kinetic energy of the photo electrons emergingfrom a cesium surface due to the action of light with a wave length of3700 A would be 3.31..9 or 1.4 electron volts. However, the averageenergy of these photo electrons would be about one-half this value or0.7 electron volt (see Flgure 3.1 of Photoelectric Phenomena by Hughesand DuBrldge, Published 1932 by McGraw-Hill Book Co., New York, N. Y.),which is much greater than the average energy of thermionic electrons.

By reason of this difference in energy, the photo electrons will be ableto pass a grid of predetermined negative voltage with respect to thecathode whereas the thermionic electrons will be hindered from passing.The voltage of the grid should be equal to the average voltage of theemitted thermionic electrons.

Obviously many modifications and variations'of the invention ashereinabove set forth may be madewithout departing from the spirit andscope thereof and only such limitations should be imposed as areindicated in the appended claims.

I claim:

1. A multi-electrode discharge device comprising in a vacuum envelope aphoto-emissive cathode and an anode at opposite ends of a path ofelectron travel starting at the cathode, at least one intermediatetarget electrode positioned along said path between the cathode andanode, said target electrode having a sensitized side on which itreceives primary electrons which reach it moving along a portion of saidpath in certain incident directions and from which it responsively emitssecondary electrons along another portion of said path in differentdirections than the incident ones, a velocityfilter-grid positionedathwart said path, said grid being the electrode of the device which isnearest-adjacent the side of the cathode which faces along said path,and terminal means for applying uniformly to said grid a potential whichis negative with respect to the cathode to establish a voltagedifference therewith equal to the average velocity of thermionicelectrons emitted from the cathode.

2. A photo-multiplier discharge device comprising in a vacuum envelope acathode having a photo-emissive surface, an anode at the end of a pathof electron travel starting at said surface, an intermediate targetelectrode having a secondary-emitter surface positioned to receivephoto-electrons from said cathode surface arriving in certain incidentdirections along a portion of said path and to give off secondaryelectrons along another portion of said path in different directionsthan the incident ones, a velocity-filter grid positioned athwart saidpath intermediate said surfaces of the cathode and the target electrode,and terminal means for applying uniformly to said grid a potential whichis negative with respect to the cathode to establish a voltagedifference therewith to prevent any significant transfer from thecathode to the target electrode of electrons having less than apredetermined average initial velocity.

3. A photo-electric device for translating light scintillations intoelectrical pulses with a minimum of back ground thermal noise comprisinga vacuum envelope containing a photo-emissive cathode having aphoto-emissive surface facing along a path of electron travel, saidsurface being subject to the spurious emission of thermionic electrons,an electrode for collecting electrons reaching the end of said path toprovide an output current, a velocity-filter grid positioned athwartsaid path, and a voltage source connected between said grid and cathodefor polarizing the former negatively with respect to the latter toprevent a larger percentage of the thermionic-electrons than of thephoto-electrons emitted by the cathode from moving through the grid inmoving along said path.

4. A multi-electrode discharge device comprising in a vacuum envelope, acathode having a photo-emissive surface facing along a path of electrontravel, an electron-collecting anode at the end of said path, anintermediate target-electrode having a secondary emitter surfacepositioned to receive electrons from said surface of the cathode and togive off secondary electrons in directions along said path toward theanode, a first velocity-filter grid positioned athwart said path, saidgrid being the electrode of the device which is nearest adjacent to saidsurface of the cathode, first terminal means for applying to said grid apotential which is sufficiently negative with respect to the cathode toprevent a larger percentage of the thermionic-electrons than of thephotoelectrons emitted from the cathode to pass through the grid to saidfirst target electrode, a second velocity-filter grid positioned athwartsaid path, said second grid being the electrode of the device which isnearest adjacent to said secondary emitter surface, and second terminalmeans for applying to said second grid a potential which is sufficientlynegative with respect to the target electrode to prevent a largerpercentage of the thermionicelectrons than the secondary-electrons givenoff thereby to pass through the second filter grid in moving along saidpath.

5. Electrical apparatus comprising: a photoelectric device includingwithin an hermetically sealed envelope a photo-emissive cathodepositioned to receive light through a wall of the envelope from anexternal source thereof, an anode for collecting electrons moving alonga path which extends from said cathode to said anode and a velocityfilter means positioned athwart said path in spaced and insulatingrelationship to said cathode; and a voltage source connected betweensaid cathode and said velocity filter means for establishing a negativeelectric field therebetween to impede the progress along said path ofmore thermal than photo-electrons.

6. In a scintillometer the improvement wherein the photo-electric devicecomprises a photo-emissive cathode for receiving light from theluminophor of the scincillometer, an anode for collecting electronsmoving along a path which extends from said cathode to said anode, andmeans, including a velocity-filter grid extending athwart said path in aregion thereof intermediate said cathode and anode and a source ofdirect voltage with its positive and negative poles respectivelyconnected to said cathode and grid, for more greatly reducing the numberof thermal electrons which can move through said region along said paththan of other, higher-energy electrons.

6 References Cited in the file of this patent UNITED STATES PATENTS1,898,080 Culver Feb. 21, 1933 2,135,615 Farnsworth Nov. 8, 1938 102,227,030 Schlesinger Dec. 31, 1940 2,234,801 Gorlich Mar. 11, 19412,256,300 Van Mierlo Sept. 16, 1941 2,305,179 Lubszynski Dec. 15, 19422,401,734 Janes June 11, 1946 16 2,517,404 Morton Aug. 1, 1950

